In the copolymerization of 1,3-butadiene and isoprene with unmodified neodymium catalysts, the 1,3-butadiene polymerizes about 19 times faster than the isoprene. For this reason, such copolymers do not have a random distribution of monomers. One end of the polymer chains contain mostly repeat units which are derived from butadiene (which polymerized faster) and the other end of the polymer chains contain mostly repeat units which are derived from isoprene (which polymerized slower). As the polymerization proceeds, the availability of butadiene monomer for polymerization diminishes leaving more and more isoprene to polymerize subsequently. This causes such isoprene-butadiene rubbers to be tapered.
U.S. Pat. No. 4,663,405 discloses that conjugated diolefin monomers can be polymerized with a catalyst system which is comprised of (1) an organoaluminum compound, (2) an organometallic compound which contains a metal from Group III-B of the Periodic System, such as lanthanides and actinides, and (3) at least one compound which contains at least one labile halogen atom. U.S. Pat. No. 4,663,405 also discloses that the molecular weight of the polymers made with such catalyst systems can be reduced by conducting the polymerization in the presence of a vinyl halide. However, its teachings do not specifically disclose copolymerizations of isoprene with butadiene and do not provide any technique for making the isoprene monomer polymerize at a rate which is similar to that of the butadiene monomer. Thus, its teachings do not provide a technique for synthesizing random, non-tapered isoprene-butadiene rubbers with catalyst systems which are comprised of (1) an organoaluminum compound, (2) an organometallic compound which contains a metal from Group III-B of the Periodic System, such as lanthanides and actinides, and (3) at least one compound which contains at least one labile halogen atom.